73-87chevytrucks.com

73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: 87becky on April 19, 2014, 12:15:48 AM

Title: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 12:15:48 AM
Hey all. I've posted before about electrical and wiring and figured it out with help to the diagrams on this forum and from all you here that helped. We'll I'm back with a new problem that I haven't had before. All started when my ICM(ignition control module) went out while wheeling. Got it running a few time but died and wouldn't do anything at all. Got it home and looked for any wires that could have shorted anything out.  I pulled up my main harness that runs behind the distributor and as I did this it was hard to pull up. I got it up and I get two black wires that were spliced as if they went to something.  I thought they got stuck so it shorted something out. Didn't do anything. Tried grounding them out on the fire wall and put them together and nothing. No power in the wires either. Got it home and changed the module from my old distributor and fired up as it always did. Bought a new module to be safe and timed it at 2 degrees advanced. Other friends with the same motor run 4 degrees advanced but mine was set this way temporarily. My issue I'm having is when I accelerate to around 2500 rpm or give it a quick rev it stutters, coughs,misfires and sometimes gives a backfire. I'm questioning it's timing maybe but not likely. I'm now down to it being the erg and that's what those two black wires were. One wire measures 74 ohms resistance to a ground and the other wire gives me -0.01 ohms resistance. I've tried grounding the wire with a lot of resistance but still does nothing. I need major help with this. I'll be testing my erg in a few days cause I'm not home now but if anyone can help please do. Thanks
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 1979C20 on April 19, 2014, 01:16:06 AM
On a TBI motor with Electronic Spark Timing, you unplug the Tan/black stripe wire next to the brake booster to disable the EST and time the motor to 0°. Then plug the EST wire back in. If a TBI motor is not at 0° you have problems with acceleration because the computer takes over advance/retard of timing from 0. When you set it to anything other than 0 you confuse the system.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 10:25:22 AM
I did unplug the black/tan wire to time it and shut the truck off and fired it back up and is fine. My guessing is the two black wires i pulled from behind the motor is causing this issue. Possible that they control my erg solenoid and not letting it operate properly and that's where I get it stumbling and what not on acceleration
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: bake74 on April 19, 2014, 11:08:13 AM
     Did you trace the 2 black wires to see where they go and what they are for ?  In the tech section there are the manuals for our trucks along with the wiring diagrams if you do not have them.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 11:33:03 AM
I haven't had much time to trace them all the way to where they go. I've searched the tech section for the wires but not much luck. My former high school auto teacher said they sound like they're ground wires that go on the back of the block. Tried grounding them out and nothing changed. I also don't see wires that ground to the block in the tech section that can relate to these wires
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: dvdswan on April 19, 2014, 11:43:53 AM
just remember grounds need to be clean; no rust, paint, or corrosion.  any of those will add resistance and affect how voltage passes through the circuit.  something that requires 12vdc +/- .5 and say corrosion causes a .7 vdc drop the circuit either won't work or operate erratically.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 11:48:08 AM
I made them ground on clean metal. Even wire brush it to be sure it's good and it was. But again I'm not sure if they are suppose to ground out or if they go to something else. No power to them even with key on so I assume they should be grounded
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: dvdswan on April 19, 2014, 11:58:15 AM
did you by chance try the timing a 4* like your friends vehicles?
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 12:02:28 PM
No I haven't done that yet. I've always been told TBI motors are picky with timing that if it's not perfect it won't run right. But funny thing is I timed it without unplugging the black/tan wire two years ago and has ran fine everywhere it's been. So I can but can't say 2 degrees off is going to make it stumble on acceleration like it's doing.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: dvdswan on April 19, 2014, 12:09:50 PM
so the 2 black wires, do you know what there were connected to on at least one end?  you said they are in the main harness behind the distro, did they go to the distro?  did you break the wires when you pulled them up?  what year are you working with?
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: rich weyand on April 19, 2014, 12:11:08 PM
What kind of plugs and wires are you running?  How old are they?

When you open the throttle, the engine gets a denser charge.  Denser charge is harder to ignite.

Sounds like something in the ignition is on the edge.  When you give it a denser charge, it can't ignite it.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 12:14:41 PM
No I don't know what they were connected to. They were spliced as if there were some type of connectors on them but no clue as to what. They didn't go to the dist. that I could see. It don't wanna say they broke but more as pulled out of something cause they were spliced. It's a 1987 r30 TBI 350 to be exact.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 12:17:47 PM
What kind of plugs and wires are you running?  How old are they?

When you open the throttle, the engine gets a denser charge.  Denser charge is harder to ignite.

Sounds like something in the ignition is on the edge.  When you give it a denser charge, it can't ignite it.


The plugs and wires are about a year and half old. New dist cap too I put on cause the cap I had looked burned inside on the terminals. What could be on edge? Could dense charge mean something with the erg not functioning properly?
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: dvdswan on April 19, 2014, 12:20:30 PM
you said you grounded them to the firewall, did you try grounding to the engine block?

Rich, I'm confused by a "denser charge",  what does that mean?
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 19, 2014, 03:34:43 PM
you said you grounded them to the firewall, did you try grounding to the engine block?

Rich, I'm confused by a "denser charge",  what does that mean?

I grounded the wire with 74 ohms resistance to the block and left the other wire alone. Still had the same problem
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: rich weyand on April 20, 2014, 12:22:38 AM
Rich, I'm confused by a "denser charge",  what does that mean?

At lower throttle settings, the intake manifold has significant vacuum as the engine keeps pumping air and the throttle plate is at least partially closed.  The pressure inside the manifold that feeds into the intake valve is less than atmospheric pressure.  This is a less dense charge, and ignites easily.

At higher throttle settings, the intake manifold is closer to atmospheric pressure because, even though the engine is pumping, the throttle plate is substantially open.  When the intake valve opens, this denser mixture enters the cylinder.  This denser charge is harder to ignite than the less dense charge that results from lower throttle settings.

When you lose power on high throttle settings, it may be because the ignition system is not providing hot enough spark to ignite the denser charge that results from less vacuum in the manifold, even though it can ignite the easier, less dense charge of lower throttle settings.  The ignition system is "on the edge" of not igniting the charge at lesser throttle settings, and at higher throttle settings it can't always ignite the charge and you have intermittent misses on one or more cylinders.  You might not notice such a miss at higher rpms that would be very apparent at idle.  That unburned charge from that miss is going to go into the hot manifold/header, and be followed up with still-burning gasses from another cylinder firing, and ignite in the manifold/header, giving you an after-fire (like a back-fire, but in the exhaust, not coming "back" through the carb).  Instead of a boom under the hood, you will get a boom from underneath the cab floor.

That's why I ask what kind of plugs and what kind of wires, and how old are they.  I think the AC Delco R45TS is the proper plug for all the 73-87 SBCs.  For wires, since those are already resistor plugs, you should run helical-wind (sometimes called spiral-wind) metal conductor wires.  Taylor has the reputation for good helical-wind metal-conductor wires.  I have a set on the way for mine (ordered them from Summit tonight for $60), given that I am experiencing an occasional after-fire when climbing some of the steep hills around here, with long pulls at high throttle settings.  I think my wires aren't giving me full spark any more.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: dvdswan on April 20, 2014, 01:04:02 PM
that is a great explanation Rich.  thank you.

87becky did you by chance use the new dielectric grease on the ICM when you replaced it?
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 1979C20 on April 20, 2014, 03:32:39 PM
I hate to sound like a broken record, but on a Chevy TBI engine, the timing needs to be at ZERO. Having the timing advanced or retarded will cause problems, including hesitation, bogging or even misfire. Unplug the tan/black wire, time the engine to ZERO degrees and reconnect the tan/black wire.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 20, 2014, 04:32:50 PM
that is a great explanation Rich.  thank you.

87becky did you by chance use the new dielectric grease on the ICM when you replaced it?


I applied the special heat paste that was supplied with the new ICM but no dielectric grease. I'll be sure to apply some when I'm working on it next
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 20, 2014, 04:36:48 PM
I hate to sound like a broken record, but on a Chevy TBI engine, the timing needs to be at ZERO. Having the timing advanced or retarded will cause problems, including hesitation, bogging or even misfire. Unplug the tan/black wire, time the engine to ZERO degrees and reconnect the tan/black wire.


I'll have to try that to see if it helps. But like I said earlier the timing wasn't perfect before and it ran fine for the past 2 years so I don't really see 2 degrees advanced making it do what it's doing but I'll try and see what happens.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: dvdswan on April 20, 2014, 05:42:06 PM
that is a great explanation Rich.  thank you.

87becky did you by chance use the new dielectric grease on the ICM when you replaced it?


I applied the special heat paste that was supplied with the new ICM but no dielectric grease. I'll be sure to apply some when I'm working on it next

that's what I meant.  I thought it was dielectric grease of some sort.
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: bd on April 20, 2014, 06:50:41 PM
As long as the base timing is between TDC and 6° BTDC, and the engine doesn't ping, timing isn't likely an issue.  There is no crank or cam position sensor on this motor, hence the timing map is merely offset by the amount of base timing.  If the engine becomes over advanced to the point of detonation, knock will cause EST to retard the timing and degrade engine output.

Does the SES light in the dash illuminate when the ignition is first switched on?  Are there any trouble codes stored?  Have you checked fuel pressure?  When was the last time the fuel filter was replaced?  Does the truck still have its catalytic converter?  Pull the plug wires off the distributor cap and coil and look up inside the boots.  Do any of them appear sooty or discolored?  Do the same thing at the spark plug ends.

If you think EGR is the problem, just unplug the vacuum line from the valve and go for a test drive.  Other than ping, does the hesitation/backfire disappear?

-------------------------------

Post a pic of the 'black wires' and where they exit the harness.  Elaborate on...

...They were spliced as if there were some type of connectors on them....  I don't wanna say they broke but more as pulled out of something cause they were spliced....

            ...Are the wires solid black or black with a white stripe?
Title: Re: Chevy TBI 350 stumbles on acceleration
Post by: 87becky on April 22, 2014, 11:54:48 PM
As long as the base timing is between TDC and 6° BTDC, and the engine doesn't ping, timing isn't likely an issue.  There is no crank or cam position sensor on this motor, hence the timing map is merely offset by the amount of base timing.  If the engine becomes over advanced to the point of detonation, knock will cause EST to retard the timing and degrade engine output.

Does the SES light in the dash illuminate when the ignition is first switched on?  Are there any trouble codes stored?  Have you checked fuel pressure?  When was the last time the fuel filter was replaced?  Does the truck still have its catalytic converter?  Pull the plug wires off the distributor cap and coil and look up inside the boots.  Do any of them appear sooty or discolored?  Do the same thing at the spark plug ends.



Ok, so today I got to get sometime with the truck and I found a few things. nothing major but good to know. SES light does come on when first switched on. No trouble codes stored. fuel pressure could be a possibility but I wanna say its not cause it can supply fuel up to 4000 rpm. Ive  reved it more in the past with the set up I have so its not likely fuel pressure but I will be testing that soon. fuel filter was replaced when I put this pump in about 6-8 months ago.  the wires are fine but it may be possible theyre breaking down inside but they are fairly new and ill be checking that tomorrow. I unhooked the egr line and nothing changed or made a difference. The two wires are solid black. As I was told solide black is just ground. so to day I put them together and grounded them out to the blok. before this my truck was running in an open loop with the O2 sensor running lean and O2 crossings weren't up where they should be. once I grounded these wires everything was normal and in perfect order. the truck is still doing what it has been doing. since I bout a ICM from autozone im going to a GM dealership to buy a good one. ill also be testing the module I have at the auto parts store to see what it does. I tried posting the pic for the wires but I cant get it to work